1. Field of the Invention
The present invention relates to two-dimensional chromatography. Specifically this invention relates to devices and method for performing two-dimensional liquid or gas chromatography with partial modulation.
2. Description of the Related Art
Two-dimensional chromatography is known in the art. The fields of both gas and liquid chromatography utilize two-dimensional separation techniques to analyze sample matrices contained in a sample analyte.
Comprehensive two-dimensional liquid or gas chromatography utilizes two orthogonal columns connected in series to separate compounds within a sample. The term “orthogonal” as used herein means that the columns separate compounds within a sample based on two different properties of the compounds. Ideally, the different properties are independent of each other, resulting in a lack of correlation between the retention time of the compound in the first column and its retention time in the second column. The stationary phase in the second column should yield a faster separation than that of the first column. In the prior art, a sample is injected into the injection port and integrated with the mobile phase. The mobile phase is necessary to introduce and transport the sample through the column. In gas chromatography, the mobile phase is generally an inert gas and is often referred to as the carrier gas. In liquid chromatography, the mobile phase is a liquid of low viscosity and is often referred to as the carrier fluid. Injection of the sample may be by a syringe or operation of a valve or valves connected to a source or loop, among other methods. As the mobile phase transports the sample through the first column, the compounds in the sample are separated based on a first property. In the prior art, the first-column separated sample-bearing carrier exits the first column and is thereafter trapped and held by operation of a modulator, which releases the first column separated sample-bearing carrier in “plugs” or “packets” to the second column. The period during which a packet of first column separated sample-bearing carrier enters the second column is typically measured in seconds and is referred to as the secondary retention time. A detector at the exit of the second column measures the intensity of compounds in each packet at the conclusion of the second separation.
Primary retention time and secondary retention time identify each compound in the sample-bearing carrier in three dimensions. Modeling of the data with intensity in three-dimensions displays quantitative and qualitative properties of the compounds within the sample.
In prior art two-dimensional gas chromatography, the sample-bearing carrier was fully modulated. U.S. Pat. No. 5,196,039 issued to Phillips et al. on Mar. 23, 1993 discloses a thermal modulator device and method of performing comprehensive multi-dimensional chemical separation using a first dimension of a two-dimensional chromatograph to generate a chromatogram in a time comparable to or even faster than prior practice while the second dimension generates multiple chromatograms each in a time comparable to the fastest prior art chromatography. The transfer of sample portions from the first dimension to the second dimension is by any one of several sample-bearing carrier modulation techniques wherein portions of sample-bearing carrier are accumulated between the first and second dimensions and transferred as very sharp concentration pulses.
An article entitled, “Time-resolved Cryogenic Modulation for Targeted Multidimensional Capillary Gas Chromatography Analysis” by Philip J. Marriott et al. was published in the Journal of Chromatography in 2000. The article discloses a method incorporating two directly coupled columns and employing a longitudinally modulated cryogenic trap located between the columns. A method termed “selected zone compression pulsing” is used. All of the first column effluent is passed to the second column. The times at which the modulation of the trap is performed determines which target solutes will be selected for enhanced separation, allowing almost instantaneous separation of selected zones in the second column.
U.S. Pat. App. Pub. No. US. 2002/0148353 by Seeley published on Oct. 17, 2002 discloses a two-dimensional gas chromatograph with a primary column and dual secondary columns. Flow rates in the primary column are less than those in the secondary column due to an accumulation valve. The accumulation valve operates to accumulate the sample as it exits the primary column, and introduce the accumulated sample to the dual secondary columns. Typically the ratio of the combined dual secondary columns flow capacity to the primary column flow capacity is approximately between 10 to 1 and 30 to 1.
U.S. Pat. No. 6,007,602 issued to Ledford, Jr. et al. on Dec. 28, 1999 discloses an apparatus and a method for forming a chemical modulation of a substance present in a fluid stream. The apparatus utilizes a movable device, such as a movable heater, to induce changes in the retention of a chemical substance flowing through the modulator tube.
U.S. Pat. No. 6,702,989, issued to Sacks et al. on Mar. 9, 2004 and U.S. Pat. Nos. 6,706,534, and 6,706,535 both issued to Sacks et al. on Mar. 16, 2004 disclose a gas chromatography system having a computer-controlled pressure controller that delivers pressurized pulses to a column junction point of two series-coupled columns having different stationary-phase chemistries. Each pressurized pulse causes a differential change in the carrier gas velocities in the two columns, which lasts for the duration of the pressurized pulse.
Comprehensive 2-D chromatography with full modulation of the sample-bearing carrier is not without drawbacks as to temperature, size and power requirements of equipment, and time required for secondary dimension analysis. Each column is operated throughout a temperature range, which may be beyond the range of the modulator. In such cases, the sample-bearing carrier is removed from the higher-temperature environment of the first column to a second environment where full modulation occurs, then reintroduced to the higher temperature environment to pass through the second column. Such cooling and heating may alter the compounds within the sample-bearing carrier, skewing the results. Such a full modulator may require cryogenic cooling, restricting the size and portability. Finally, the secondary retention time for full modulation should be long enough for a full separation in secondary dimension.
It would be an improvement to the art to be able to sufficiently modulate an analyte-bearing sample to permit comprehensive 2-D liquid or gas chromatography which would not alter the chemical properties of primary separation and which would permit a shorter secondary retention time.